In 3GPP LTE (3rd Generation Partnership Project Long-term Evolution), studies are conduct to use a ZC sequence as an RS in uplink. This ZC sequence is a kind of CAZAC sequence and represented by the following equation 1.
                    (                  Equation          ⁢                                          ⁢          1                )                                                                                  a                          r              ,              m                                ⁡                      (            k            )                          =                  {                                                                                                                                                                  exp                          ⁢                                                      {                                                                                                                                                                -                                    j2π                                                                    ⁢                                                                                                                                          ⁢                                  r                                                                N                                                            ⁢                                                              (                                                                                                                                            (                                                                              k                                        +                                                                                  m                                          ⁢                                                                                                                                                                          ⁢                                          Δ                                                                                                                    )                                                                        2                                                                    2                                                                )                                                                                      }                                                                          ,                                                                                                            when                        ⁢                                                                                                  ⁢                        N                        ⁢                                                                                                  ⁢                        is                        ⁢                                                                                                  ⁢                        even                                                                                                                                                                          exp                          ⁢                                                      {                                                                                                                                                                -                                    j2π                                                                    ⁢                                                                                                                                          ⁢                                  r                                                                N                                                            ⁢                                                              (                                                                                                                                            (                                                                              k                                        +                                                                                  m                                          ⁢                                                                                                                                                                          ⁢                                          Δ                                                                                                                    )                                                                        ⁢                                                                          (                                                                              k                                        -                                                                                  m                                          ⁢                                                                                                                                                                          ⁢                                          Δ                                                                                +                                        1                                                                            )                                                                                                        2                                                                )                                                                                      }                                                                          ,                                                                                                                                      when                          ⁢                                                                                                          ⁢                          N                          ⁢                                                                                                          ⁢                          is                          ⁢                                                                                                          ⁢                          odd                                                ,                                                                                            ⁢                                                                  ⁢                k                            =              0                        ,            1            ,            …            ⁢                                                  ,                          N              -              1                                                          [        1        ]            Here, N is the sequence length of the ZC sequence and r is the ZC sequence number (i.e. sequence index), and N and r are coprime. Reasons to study a ZC sequence as an RS include constant frequency response characteristics, good auto-correlation characteristics and good cross-correlation characteristics between sequences (between sequences of different sequence numbers).
Further, if the sequence length N of a ZC sequence is a prime number, N−1 sequences of good cross-correlation characteristics can be generated. At this time, the cross-correlation between sequences (e.g. different ZC sequence numbers r=1 and r=5) is fixed at √N. If the sequence length N is not a prime number, the maximum value of cross-correlation between sequences is equal to or more than √N.
Further, ZC sequences can be used as a cyclic shift sequence (hereinafter the cyclic shift sequence number is m). Here, a cyclic shift sequence refers to a ZC sequence having the same sequence number r and applied a different amount of cyclic shift (hereinafter simply “the amount of shift”), which can be generated by cyclically shifting a ZC sequence by an amount of shift in the time domain. For example, FIG. 1 shows a ZC sequence (m=0) of a sequence length N=12 and amount of shift Δ=6, and its cyclic shift sequence (m=1). In the figure, the ZC sequence (m=0) is configured in ascending order from a(0) to a(11), and its cyclic shift sequence (m=1), which is the ZC sequence (m=0) cyclically shifted by Δ(=6) symbols, is configured in ascending order from a(6) to a(11) and from a(0) to a(5).
Next, the processing of an RS in the receiver will be explained. When the above two sequences are multiplexed upon the same time and the same frequency, in the correlation result in the correlator of the receiver, the sequence of m=0 and the sequence of m=1 each produce a high correlation value (hereinafter “correlation value peak”) at timings the amount of shift Δ apart (see FIG. 2). The amount of shift Δ is set greater than the maximum delay time of delay waves, so that the correlation value peak of each cyclic shift sequence is generated only within the range of the amount of shift Δ. Therefore, by extracting only the correlation value from the period (i.e. the window part) where the correlation value of the desired cyclic shift sequence is present, as shown in FIG. 3, it is possible to detect the correlation results of individual cyclic shift sequences separately. Consequently, cyclic shift sequences can be used as orthogonal sequences on conditions that the correlation values at respective amounts of shift are present in separate windows.
Correlation calculation for cyclic shift sequences is generally processed in the frequency domain, and the correlator shown in FIG. 2 performs DFT (Discrete Fourier Transform) processing, division by a ZC sequence (m=0) and IFFT (Inverse Fast Fourier Transform) processing.
FIG. 4 is a block diagram showing the configuration of a general receiver. In this figure, DFT section 11 performs DFT processing on a received RS, and outputs the RSs after the DFT processing to subcarrier demapping section 12.
Subcarrier demapping section 12 extracts, from the RSs outputted from DFT section 11, the parts corresponding to the transmission band, and outputs the extracted RSs to division section 13.
Division section 13 divides the RSs outputted from subcarrier demapping section 12 by a ZC sequence (m=0), and outputs the correlation values, which are the division results, to IDFT section 14.
IDFT section 14 performs IDFT processing (Inverse Discrete Fourier Transform) on the correlation values outputted from division section 13, and outputs the correlation values after the IDFT processing to masking processing section 15.
Masking processing section 15 extracts only the correlation value in the period (i.e. the window part) where the correlation value of the desired cyclic shift sequence is present, from the correlation values outputted from IDFT section 14, and outputs the extracted correlation value as a channel estimation value, to DFT section 16.
DFT section 16 performs DFT processing on the correlation value outputted from masking processing section 15, and outputs the signal after the DFT processing.
F(X) represents the X-th symbol generated by performing DFT processing on a ZC sequence (or its cyclic shift sequence). Alternately, F(X) represents the X-th symbol generated by generating a ZC sequence directly in the frequency domain.
Meanwhile, amongst the RSs used in uplink, studies are underway to transmit the reference signal for channel estimation used to demodulate data (hereinafter “DM-RS,” which stands for demodulation reference signal) in the same band as the data transmission bandwidth. That is, when the data transmission bandwidth is a narrow band, it naturally follows that the DM-RS is transmitted in a narrow band. For example, if the data transmission bandwidth is one RB (resource block), the DM-RS is also one RB, and, if the data transmission bandwidth is two RBs, the DM-RS is also two RBs. Here, the RB is the unit of radio frequency allocation in the frequency domain, and is formed with, for example, one or a plurality of frequency subcarriers (hereinafter “subcarriers”).
If one RB is formed with 12 subcarriers, a ZC sequence having a sequence length N of 12 symbols (where N is not a prime number), or 11 or 13 symbols (where N is a prime number), is used for the DM-RS using one RB, and a ZC sequence having a sequence length N of 24 symbols (where N is not a prime number), or 23 symbols or 29 symbols (where N is a prime number), is used for the DM-RS using two RBs. Here, when ZC sequences having sequence lengths of 11 symbols and 23 symbols are used, DM-RSs of 12 symbols and 24 symbols are generated by performing cyclic extension. Further, when ZC sequences having sequence lengths of 13 symbols and 29 symbols are used, DM-RSs of 12 symbols and 24 symbols are generated by performing truncation.
Further, to reduce inter-cell interference between RSs, studies are underway to assign different ZC sequences for the DM-RS between cells. However, a shortage in the number of orthogonal sequences available for use in narrowband transmission or a shortage in the number of quasi-orthogonal sequences having low cross-correlation, has been raised as an issue. If the times to take to transmit DM-RSs are respectively the same, the sequence lengths of DM-RSs (the number of symbols) decrease in the narrowband transmission. For the reason, the number of sequences (N−1) decreases, and the distance between cells where the same ZC sequence is used decreases. As a result, the influence of interference between cells by DM-RSs increases and the accuracy of channel estimation is damaged significantly.
In order to make the distance longer between cells using the same ZC sequence in narrowband transmission, Non-Patent Document 1 and 2 propose assigning different cyclic shift sequences m of the same sequence number r between cells where synchronization of transmission frame timing (e.g. cells belonging to the same base station) or time synchronization between base stations is established (see FIG. 5). For example, in cells belonging to the same base station (e.g. the three cells shown by oblique lines in FIG. 5), ZC sequences having the same sequence number r are used. In cell #1, the cyclic shift sequences m=1 and 4 are used, in cell #2, the cyclic shift sequences m=2 and 5 are used, and in cell #3, the cyclic shift sequences m=3 and 6 are used. As a result, it is possible to make the distance longer between cells using ZC sequences of the same sequence number r. Further, DM-RSs under the same base station are orthogonal (i.e. ZC sequences of cyclic shift sequences), not quasi-orthogonal (i.e. ZC sequences of different sequence numbers r), so that it is possible to reduce interference from nearby cells by DM-RSs and improve the accuracy of channel estimation.    Non-Patent Document 1: Motorola, R1-062610, “Uplink Reference Signal Multiplexing Structures for E-UTRA,” 3GPP TSG RAN WG1 Meeting #46 bis, Seoul, Korea, Oct. 9-13, 2006    Non-Patent Document 2: Panasonic, R1-063183, “Narrow band uplink reference signal sequences and allocation for E-UTRA,” 3GPP TSG RAN WG1 Meeting #47, Riga, Latvia, Nov. 6-10, 2006